Aqueous polyurethane dispersions have long been known as binders in the preparation of coatings on metallic, mineral, wood or plastic substrates.
For example, polyurethane resin dispersions having a segment structure of the polymer backbone which leads to a two-phase or more-than-two-phase character and, associated therewith, to excellent elastic properties of the resins have been described (cf. e.g.: Dr. H.-P. Klein and Dr. M. Schwab, Kunstharz-Nachrichten 29, pages 38 to 42, 1993). In these dispersions, for example, long-chain polyether-, polyester- or polycarbonate-polyol units form the soft segments and urethane units, formed from diisocyanate, glycol and dihydroxycarboxylic acid, the hard segments.
Moreover, oxidatively drying polyurethane-alkyd resin hybrids are known, which carry unsaturated fatty acids as substituents and which are employed, in particular, in the field of corrosion protection.
DE-A 3901190 describes alkyd resins based on polyesterpolyols which are reacted with a 2,2-bis(hydroxymethyl)alkanecarboxylic acid and a diisocyanate component in a one-pot reaction so as to give a resin having a weight-average molecular weight Mw of from 10,000 to 70,000 daltons, an acid number of from 18 to 36 mg of KOH/g, which still contains free hydroxyl groups but no longer contains isocyanate groups, with from 45 to 100% of the carboxyl groups introduced into the resin being converted to carboxylate groups by addition of a base.
DE-A 4326270 embraces two-component polyurethane coating compositions, especially for the preparation of multicoat finishes in the automotive sector, which comprise aqueous dispersions of water-soluble or water-dispersible binders having isocyanate-reactive groups, as well as polyisocyanates as crosslinking components. Examples described of suitable binders are alkyd-free anionic polyesters, prepared from polyalcohols and polycarboxylic acids having hyroxyl numbers of between 10 and 200 mg of KOH/g, acid numbers of between 10 and 100 mg of KOH/g and number-average molecular weights Mn of between 1000 and 100,000 daltons.
DE-A 4328092 describes emulsifier-free aqueous coating compositions which contain as binder a combination of (A) a mixture of one or more water-dilutable polyurethane resins and/or alkyd-urethane resins, which may contain unsaturated fatty acid residues as substituents, with one or more water-dilutable (meth)acrylate copolymers, polyurethane resins and/or polyester resins, and (B) one or more amino resins and one or more blocked polyisocyanates. The binders described have acid numbers of between 10 and 100 mg of KOH/g, hydroxyl numbers of between 10 and 150 mg of KOH/g and number-average molecular weights of between 2000 and 10,000 daltons.
DE-A 4226243 describes water-dilutable coating compositions comprising (A) film-forming polymerpolyols in the form of aqueous dispersions of fatty acid-modified polyurethane resins and/or fatty acid-modified polyester resins (alkyd resins) with saturated and unsaturated fatty acid residues, and (B) polyisocyanates having more than one free isocyanate group, with or without the addition of one or more organic solvents and of the customary paint additives, such as pigments, fillers or rheological auxiliaries, as two-component coating compositions.
In the case of so-called do-it-yourself decorating paints, which are employed predominantly in the coating of wood, plastic or metal substrates, there is a need for low-solvent one-component systems which ensure rapid curing of the coating at room temperature. Moreover, such decorating paints should be low in yellowing, stable to weathering, mar-resistant and of high gloss, and should have a high resistance to water. In particular, the resistance to yellowing, which is essentially induced by oxidative processes, and the low solvent content in the coating material, which is required as a result of increasingly more restrictive environmental protection regulations, are of outstanding importance for decorating paints.